1. Support from the government.
Countries who gamble a lot has solid laws protecting their players so gamblers in those countries can just freely gamble as much as they want without having to worry about their rights. This doesn't mean that the countries have very free and loose rules but it just means that they have clear frameworks. Some other countries have some limitations but since their rules are clear, many still gamble.
Where gambling is legal people would freely gamble since it is not against the law of the country and this means that betting sites can freely advertise their platform which will motivate and trigger so many people to gamble. So, obviously there are more gamblers in countries where gambling is acceptable. I believe that people would even travel to such countries just to gamble and enjoy certain kinds of casino. A core gambler would be skeptical traveling to a country that prohibits gambling, that would be a limitation to his freedom. Where gambling is prohibited there would be very few gamblers and the gamblers are aware of the implication if they are caught. This fear would prevent so many from gambling and some may not have the knowledge of using softwares like VPN to access gambling platforms. Thus, legislation is decider in accessing why there are more gamblers in certain countries than others.
2. Bigger paychecks.
Not all rich people are gamblers but they certainly have more chances to gamble if they wanted to. According to data, the higher the income is in some countries the more people gamble in that country. Since the average citizen is earning a lot, even if they spend quite a significant amount of money in gambling this will still not cause financial doom.
I think that this stats is based on the value of the currency but if we go by the total number of gamblers per head then I will say that there are more gamblers in low income countries who are hoping to get a better life from gambling. No doubt, few gamblers from high income countries may be spending more in gambling compared to the so many gamblers in low income countries due to the value for money. For instance, don't be surprised that if you accumulate the amount Drake alone spends in gambling for a year it may exceed the total amount of money gotten from all gamblers from low income countries in that year. This is why stats will always show that people gamble more when they have bigger paycheck but the reality is that when people gets bigger paycheck they don't see the need for gambling and only few would be gambling for fun.